The present invention is related to processes for the removal of vanadium and/or other contaminants from residual fuel oil. More particularly, the present invention relates to supercritical extraction processes for treating residual fuel oil to achieve a higher performance in gas turbine operation.
Residual oil is the oil that remains after crude oil is distilled. It is not a uniquely characterizable fuel. The composition of the residual oil is a function of both the crude oil from which it was derived and the conditions under which it was prepared. Moreover, determination of the chemical composition of a given residual oil is a difficult task, since it composed of a wide spectrum of compounds. To avoid this characterization problem, it is common in the petroleum industry to describe oil fractions by operational definitions. For example, various fractions obtained in distillation are described by the "still" temperature at which they were drawn. This is also referred to as the "cut" temperature. Similarly, the various fractions of residual oil are described by their solubility in known solvents. In particular, the asphaltenes comprise those crude oil compounds which are not soluble in pentane. It is the treatment of this residual oil which is described herein. More particularly, it is seen below that a first part of the process of the present invention involves separation of residual oil into an oil phase and an asphaltene phase.
However, the use of residual oil in gas turbine plants, particularly gas turbine power generating plants has certain difficulties associated therewith as a result of undesirable contaminants present in the residual oil. Moreover a significant contaminant which is present in residual oil is vanadium. Vanadium is considered a contaminant since it has a highly deleterious effect on gas turbine components. Accordingly, it is desirable that the vanadium be removed from the residual oil before it is employed as fuel for gas turbine plants. At present, the problem of vanadium contamination is solved through the utilization of magnesium injection to reduce the corrosive effects of vanadium in the residual oil. It is also noted that the amount of magnesium added is proportional to the concentration of vanadium in the fuel. However, the use of magnesium as an additive results in the accumulation of ash-like deposits on interior gas turbine parts. This necessitates periodic shutdown and maintenance of the gas turbine to remove the deposits that result, from magnesium injection.
The magnesium additive is included to reduce the corrosive effects of vanadium in the residual oil. Any operation which reduces the vanadium concentration also reduces the amount of magnesium required. If a process were available to reduce the vanadium from the fuel oil, many cost factors associated with residual oil treatment would be eliminated or reduced. The present invention is directed to a process which accomplishes all of those objectives associated with vanadium removal.
The vanadium is chemically bound in some of the organic molecules in the residual oil, and thus is soluble in the oil. Typically, 50% of the vanadium is incorporated in the oil as a porphyrinic structure. The basic porphyrinic structure has a molecular weight of approximately 400, and can exist as monomer or polymer or associate with other forms, and thus can be incorporated into very large structures. Similarly, the non-porphyrinic forms span a very wide range of molecular weights. L. W. Corbett has performed an analysis of the distribution of metals in a typical residual oil ("Distribution of Heavy Metals in Asphalt Residuals", A.C. Symposia, Div. of Pet. Chem., Miami Beach, Apr. 9-12, 1967). The analysis implies that it is possible to separate approximately 70% of the oil containing a very modest amount of vanadium by distillation. Also, 70% of the vanadium is associated with the asphaltenic fraction, which makes up 15% by weight of the oil. By simply deasphalting the oil, a yield of 85% with 70% vanadium removal is possible. In accordance with the present invention, this vanadium removal is achieved by the selective extraction of the vanadium bearing compounds from deasphalted oil through the utilization of supercritical fluid extraction methods.
Supercritical fluid extraction (SCFE) is a chemical engineering unit operation whose applications are rapidly growing. As the name implies, supercritical fluid extraction is an extraction process in which a fluid above its critical point is used as the solvent. Thus SCFE is in a sense, a hybrid between liquid extraction and distillation in that the fluid has the flow properties of a gas, yet the solvent properties of a liquid. Although the principals of SCFE have been known for over 100 years, the operation has only recently begun to elicit interest as its advantages have only recently been recognized. Some of these advantages are that SCFE is frequently more energy efficient, more selective, runs at lower temperatures and can exhibit a larger solubility difference over a narrower range of conditions in either distillation or extraction. The use of SCFE for refining residual oils has been employed in the past as a means to deasphalt oil. For example, supercritical propane has been used as a fluid for such a deasphalting process. See, for example, the article "Compressed Hydrocarbon Gasses as a Solvent" by T. P. Zhuze (Petroleum, London, Volume 23, page 298, 1960 ). Additionally, C. A. Irani and E. W. Funk in "Separations Using Super Critical Gasses--Recent Developments in Separation Science" (Volume III, Part A, page 171, CRC Press, West Palm Beach, Fla., 1977) have noted that vanadium concentration increases with yield, they conclude that there is no substantial advantage of this process over conventional liquid extraction for solvent deasphalting. However, the objective of the extraction processes described by Zhuze and Irani and Funk are merely directed to deasphalting without regard to vanadium content. In contrast, the present invention is directed to the utilization of solvents and conditions which are particularly directed to vanadium removal.